Privacy and seizure control circuit for a remote station dictating system

ABSTRACT

Each one of a plurality of remote dictating stations in a recording system includes a first semiconductor switch which is biased to become conductive (&#39;&#39;&#39;&#39;on&#39;&#39;&#39;&#39;) when it is connected to a first bias source in a central record/playback unit by the closing of a cradle switch when the dictator phone of that station is lifted from its cradle. The turning on of the first semiconductor switch turns on a semiconductor seizure switch which in turn activates a semiconductor motor driver switch. The semiconductor motor driver switch, when activated, energizes a relay which controls the motor of the record/playback unit. When the semiconductor seizure switch becomes conductive, a bias voltage is fed back to the activated remote dictating station to switch on a second semiconductor switch in that dictating station which supplies a bias voltage to maintain the first semiconductor switch in its conductive state. Simultaneously the turning on of the motor driver switch produces a second bias voltage which is opposite in polarity to the voltage from the first bias source and which effectively prevents the first bias source from making the first semiconductor switch in any other remote dictating station conductive and thereby blocks the activation of any other remote dictating station and the seizing of control of the record/playback unit.

United States Patent [191 Matz I PRIVACY AND SEIZURE CONTROL CIRCUIT FORA REMOTE STATION DICTATING SYSTEM Assignee:

Filed:

Appl. No.:

US. Cl. 179/100.l DR Int. Cl. H04m 11/10 Field of Search 179/6 E, 100.1DR

[56] References Cited UNITED STATES PATENTS 6/1972 Nyeetal 179/10 O.1 DR1/1973 Nye ..l79/l00.lD

Primary Examiner-Bernard Konick Assistant Examiner -Stewart LevyAttorney, g or Firm-Curtis, Morris & Sat foRIE Arthur V. Smith, Esq.

in] 3,835,261 [451 Sept. 10, 1974 switch which is biased to becomeconductive (on) when it is connected to a first bias source in a centralrecord/playback unit by the closing of a cradle switch when the dictatorphone of that station is lifted from its cradle. The turning on of thefirst semiconductor switch turns on a semiconductor seizure switch whichin turn activates a semiconductor motor driver switch. The semiconductormotor driver switch, when activated, energizes a relay which controlsthe motor of the record/playback unit. When the semiconductor seizureswitch becomes conductive, a bias voltage is fed back to the activatedremote dictating station to switch on a second semiconductor switch inthat dictating station which supplies a bias voltage to maintain thefirst semiconductor switch in its conductive state. Simultaneously theturning on of the motor driver switch produces a second bias voltagewhich is opposite in polarity to the voltage from the first bias sourceand which effectively prevents the first bias source from making thefirst semiconductor switch in any other remote dictating stationconductive and thereby blocks the activation of any other remotedictating station and the seizing of control of the record/- playbackunit.

57 ABSTRACT Each one of a plurality of remote dictating stations in 8Claims, 2 Drawing Figures a recording system includes a firstsemiconductor t i;\ Kt;-]:' i 75 USE lfi l z Y/Z/z l ,5. if? g l J I iv24 5 SWITCH l fifl 'fflt l 5 1 i I I I I g I i I g l I 5 l 1 i I a l Iom 1 l l mi 1%; I l f +24v0trs 70| I I I :1 Cate, l i l l l I l l i 4anew I I I I azataa I Kir /2 I i W4 l i w 32 5 DICTATE A? V CONTROLSPuysA K I 4( I V; t 4 010 C/RCU/TS PRIVACY AND SEIZURE CONTROL CIRCUITFOR A REMOTE STATION DICTATING SYSTEM BACKGROUND OF THE INVENTION Theinvention relates to a dictating system of the type having a pluralityof remote dictating stations for use with a central record/playback typeunit and more particularly to the privacy and seizure control for theremote dictating stations of such a dictating system.

In some types of record/playback dictating systems, for example asdisclosed in US. Pat. Nos. 3,328,536 and 3,467,790, a plurality ofremote dictating stations are interconnected to one or more centralrecord/- playback units. Each record/playback unit is, in turn,connected to a transcribing unit. Such systems permit therecord/playback reproducing unit and its connected transcriber unit tobe shared on a time basis by the remote dictating stations. In suchsystems, however, it is necessary to provide some means by which aparticular remote dictating station may engage the record/playback unitand simultaneously block the seizure of control of the record/playbackunit by any other remote dictating stations. In order to accomplish thispurpose various privacy and seizure systems and mechanisms have beendevised.

In general, the prior art systems of this type utilizeelectro-mechanical mechanisms such as relays and solenoids either at therecord/playback unit or in the remote dictating station. In one suchprior art system, for example, each remote dictating station is providedwith a solenoid activated switch. After the record/playback unit hasbeen seized by a particular one of the dictating stations, the solenoidsin each one of the remaining remote dictating stations are energized toprevent the seizure control switches in each of the remote dictatingstations from connecting the dictating stations with the seized"record/playback unit.

In all such systems of this type relatively large currents must becarried by the wires connecting the remote dictating stations to therecord/playback mechanism in order to operate the electro-mechanicaldevices involved. This requirement makes such prior art systemsincompatible for use with modern telephone wiring of the type used inmulti-frequency transistorized telephone systems which utilizerelatively light gauge wiring because of the small currents which arenormally carried by such telephone wiring. A further disadvantage ofsuch prior art systems is that they are relatively expensive, requirethe installation of bulky components, are relatively heavy and they arenoisy.

Furthermore in those prior art systems which utilize vacuum tubesinstead of electro-mechanical devices still other disadvantages arepresented. Such systems are generally not able to withstand the physicalshocks to which systems of this type are subjected and are thereforeunreliable. They also consume a relatively large amount of power andtherefore are expensive to operate on a continuous basis.

SUMMARY OF THE INVENTION The above and other disadvantages are overcomeby the present invention of a privacy and seizure circuit for use in adictation system having a plurality of remote dictating stations and acentral record/playback unit wherein the remote dictating stations ofthe invention each include a first electronic switch, first means forbiasing the first electronic switch into conductive and non-conductivestates, means, including a normally open pickup switch, for selectivelyconnecting the first biasing means to the first electronic switch,second means for supplying a bias voltage of a polarity opposite to thatof the first biasing means and a second electronic switch in each remotedictating station which is connected to the first electronic switch andto the second biasing means such that when the second electronic switchis turned on it maintains the first electronic switch in its conductiveor on state. The second electronic switch is so connected to the firstelectronic switch that when the first electronic switch is madeconductive by the closing of the pickup switch the second electronicswitch is also made conductive.

The record/playback unit of the invention includes an electronic seizureswitch which is connected to the first electronic switches in each oneof the remote dictating stations so that when the first electronicswitch in any particular remote dictating station is switched into itsconductive state, the seizure switch is also switched into itsconductive state. An electronic, motor driver circuit is connected inseries with the seizure switch to the second bias means so that when theseizure switch becomes conductive the motor driver circuit is activatedand the motor of the record/playback unit is activated.

The activation of the motor driver circuit also causes a biasvoltagefrom the second biasing means to be fed to each one of the remotedictating stations to effectively override the voltage of the firstbiasing means and thereby provide a potential reverse bias for each ofthe first electronic switches not yet in a'conductive state. If thepickup switch connected to one one of these first electronic switches isclosed, this reverse bias is applied to block the first electronicswitch connected to the closed pickup switch from becoming conductiveand thereby turning on its associated second electronic switch. In thismanner only the first remote dictating station which is activated willhave seizure control of the record/playback unit because only its secondelectronic switch remains conductive to maintain it in its conductivestate. v

One advantage of the preferred embodiments of the system of theinvention is that semiconductor electronic switches are utilized so thatonly relatively low currents need be passed through'the wires whichconnect the remote dictating stations with the record/- playback unit.This makes the system suitable for use with the low gauge wiring systemsof multi-frequency, transistorized telephone systems such as arecommonly found in modern office buildings. Furthermore the use ofelectronic circuitry allows for miniaturization of the system which isnot possible when bulky electromechanical devices of prior art systemsare used.

In prior art systems of the electro-mechanical type it is necessary toactually disconnected each of the blocked out remote dictating stationsfrom this system by complex electro-mechanical circuitry. In contrast,the present invention, because of its use of electronic switches, ,doesnot require that such unused remote dictating stations by physicallydisconnected from the circuit but instead a reverse bias voltage isapplied to the electronic switches of the blocked out devices to disablethem temporarily. They still remain physically and electricallyconnected to the record/playback unit, however, which greatly simplifiesthe construction and operation of the system.

The seizure and privacy control circuit of the present invention issuitable for use in the DICTATING AND TRANSCRIBING SYSTEM described inco pending application Ser. No. 317,928 filed Dec. 26, 1972 of which theapplication is a joint inventor, and the disclosure in that applicationis incorporated herein by reference.

I It is therefore an object of the present invention to provide a solidstate system to control the privacy and seizure functions of a remotedictator in a dictating/- playback system so that if one remotedictating station activates the system all other dictating stations areblocked out of the system.

It is another object of the invention to provide a privacy and seizurecontrol for a remote dictating station of a record/playback system whichutilizes relatively low electrical currents.

It is still a further object of the invention to provide a privacy and,seizure control circuit for use in a dictating/playback system which issimple in construction and light in weight.

The foregoing and other objectives, features,'and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of certain preferred embodiments of theinvention, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammaticrepresentational view of a typical dictating/playback system of the typefor which the circuit of the invention is intended; and

FIG. 2 is a schematic diagram of a privacy and seizure control circuitaccording to one embodiment of the invention for use in thedictating/playback system depicted in FIG. 1.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS Referring now moreparticularly to FIG. 1 a dictating/playback system is diagrammaticallyillustrated as comprising a central record/playback unit which isconnected through a multi-conductor cable 12 to a plurality of remotedictating stations illustrated generally by dictating stations 14 and 16which are connected in parallel with the record/playback unit 10. Therecord/playback unit 10 is also connected through a multi-conductorcable 18 to a transcribing station 20. At the transcribing stationmessages recorded from the dictating stations 14 or 16 on an endlessloop tape (not shown) within the record/playback unit 10 may be playedback over the transcribing station 20 to a typist. The transcribe modeof the record/playback unit 10 may be controlled by the typist through afoot pedal 22 connected to the record/playback unit through a cable 24.The playback mode of each dictating station is controlled at theparticular dictating station 14 or 16.

When any particular dictating station is being used (activated") torecord or playback messages on the record/playback unit 10, it isdesirable that the remaining remote dictating stations connected inparallel with it be blocked from controlling the record/playback unitand be unable to hear what is being recorded or played back through theactivated remote dictating station. A privacy and seizure controlcircuit for this purpose according to the invention will now bedescribed with reference more particularly to FIG. 2.

The dictating station 14, illustrated in FIG. 2 as being enclosed in adotted line, is comprised of a microphone 26 having one lead connectedthrough a capacitor 20 and a resistor 30 to the base electrode of an NPNtransistor 32. The other lead of the microphone 26 is con necteddirectly to the emitter electrode of the transistor 32 and through aresistor 34 to its base electrode. The base electrode of the transistor32 is also connected to the emitter electrode through a capacitor 36.The base electrode of the transistor 32 is connected through a biasresistor 38 to a bus biasing lead 40.

The collectorof the transistor 32 is connected directly to the baseelectrode of a PNP transistor 42. The base electrode of the transistor42 is connected through a resistor 44 to the lead and the emitterelectrode of the transistor 42 is connected directly to the lead 40.

The collector of the transistor 42 is connected directly to the base ofthe NPN transistor 46 whose collector is connected directly to the lead40 and whose emitter is connected directly to the emitter electrode ofthe transistor 32. The emitters of the transistors 32 and 46 are alsoconnected to one lead of a playback speaker 48 within the dictatingstation. The other lead of the playback speaker 48 is connected througha capacitor 50 to a common return conductor 52.

The emitter electrodes of the transistors 32 and 46 are also connectedthrough a resistor 54 to one lead of a normally open, dictate switch 56whose other lead is connected to the common return conductor 52. Aplayback/rewind control circuit 58 is connected by one lead to anexternal terminal 13a at the dictating station 14 which is connected bya wire 12a in the multiple conductor 12 to a terminal 13b at therecord/playback unit 10. The external terminal 13!? is connected to oneinput of a dictating/playback control circuit, designated generally byreference numeral 126, in the record/playback unit 10. Theplayback-rewind control circuit 58 in the dictating station 14 isconnected by separate leads to the common return conductor 52 and thejunction of the resistor 54 and the dictate switch 56. This latterconnection serves as a DC. return path.

The biasing bus lead 40 is connected through a resistor 60 to anexternal terminal 130 at the dictating station 14 which is connectedthrough a wire 12b in the multiple conductor cable 12 to a terminal 13dat the record/playback unit 10. The external terminal 130 is alsoconnected through a resistor 62 to the common return lead 52.

The common return lead 52 is connected directly to the emitter electrodeof a PNP transistor 64 whose collector electrode is connected through aresistor 66 to an external terminal 13g at the dictating station 14. Theexternal terminal 13g is connected through a wire 12d within themultiple conductor cable 12 to an external terminal 13h at therecord/playback mechanism 10. The terminal 13h is connected tothe'circuit ground within the record/playback unit 10.

The common return lead 52 is also connected through a resistor 68 to thebase electrode of the transistor 64 and to the collector electrode of anNPN transistor 70. The base electrode of the transistor 70 is connecteddirectly to the collector electrode of the transistor 64 and through aresistor 72 to the emitter electrode of the transistor 70. A capacitor74 is connected in parallel with the resistor 72. The emitter electrodeof the transistor 70 is connected to one lead of a normally open, cradlepickup switch 76 whose other lead is connected to the anode of a diode78. The cathode of the diode 78 is connected to the grounded externalterminal 13g. The normally open cradle pickup switch is closed when thehandset of the dictating station is lifted from its cradle. A capacitor80 is connected in parallel with the diode 78. The anode of the diode 78is connected through a resistor 82 to the anode terminal of a diode 84whose cathode terminal is connected through an in use" light 86 to theterminal 13g. The anode terminal of the diode 84 is also connected to anexternal terminal 13e at the dictating station 14. The terminal 13e isconnected by a wire 120 in the multiple I conductor cable 12 to anexternal terminal 13f at the record/playback unit 10.

The external terminal 13d of the record/playback unit is connectedthrough a resistor 88 to the base electrode of a PNP seizure transistor90 and through a resistor 92 to a bias source 94 of +24 volts. Acapacitor 96 is connected in parallel with the resistor 92. The emitterelectrode of the transistor 90 is connected to the bias source 94. I

The collector electrode of the transistor 90 is connected to the baseelectrode of an NPN, motor driver transisitor 100 and through a resistor102 to a bias source 104 of 44 volts. The collector electrode of thetransistor 90 is further connected to the external terminal 13f througha resistor 106 connected in parallel with a capacitor 108. The externalterminal 13f is connected to the circuit ground through a capacitor 124.

The base electrode of the transistor 100 is connected to the circuitground through a resistor 110 connected in parallel with a capacitor112. The collector electrode of the transistor 100 is connected througha resistor 114 to the bias source 94. The emitter electrode of thetransistor 100 is connected directly to the terminal 13f and to theanode of a diode 116 whose cathode is connected to one lead of the coilof a motor control relay 118. The other lead of the relay coil 118 isconnected to the circuit ground. The anode of a diode 120 is connectedto the circuit ground and the cathode of the diode 120 is connected tothe cathode of the diode 116. The purpose of the diode 120 is to providea shunt circuit for back EMF generated in the relay coil 118 whenvoltage is rapidly supplied .and discontinued through the diode 116. Themotor relay 118 opens and closes a pair of relay contacts 122 toenergize the motor (not shown) of the record/playback unit 10.

The external terminal 13d is connected to a separate input of thedictate/playback/rewind control circuit 126 and to the dictate/playbackaudio circuit 128 which are within the record/playback unit 10. Thecircuits 126 and 128 are not shown in detail since they are not directlypertinent to thepresent invention and they may comprise circuits of thetype known to those skilled in the art. As will be explained in greaterdetail hereinafter, the control circuits 56 and 58 in the dictating unit14 actiavate the circuits 126 and 128 by providing a circuit ground pathto the input leads of the circuits 126 and 128. It is to be understoodthat a plurality of dictating stations are connected in parallel throughthe multiple conductor cable 12 to the terminals 13b, 13d, 13f and 1311of the record/playback unit 10.

In operation, when the hand unit of a particular dictating station, suchas dictating station 14, is lifted from its cradle the cradle pickupswitch 76 is closed thereby supplying a 44 volt bias from the source 104through the resistors 102, 106 and 82 and the switch 76-to the emitterelectrode of the transistor 70. The 44 volt bias supplied to the emitterelectrode of the transistor also flows through the resistors 72 and 66to the circuit ground thereby developing a positive bias voltage on thebase of transistor 70 which makes it become conductive or on.

When transistor 70 becomes conductive current from the +24 volt source94 flows through the resistors 92, 88, and 62, the emitter-base junctionof the transistor 64 in parallel with the resistor 68, thecollectoremitter junction of the transistor 70, the switch 76 and thediode 78 to the circuit ground. The voltage drop developed across theresistor 68 places a negative bias on the base of the transistor 64 withrespect to its emitter thereby making it conductive. When transistor 64becomes conductive it supplies a positive bias to the base of transistor70 from the junction of the resistors 62 and 68 and thus the transistors64 and 70 lock up on each other in the conductive condition.

The flow of bias current from the +24 volt source 94 through theresistor 92 as described above also develops a negative bias voltage onthe base electrode of the transistor thereby turning it on. Whentransistor 90 thus becomes conductive, current from the +24 volt source94 flows through the emitter-collector junction of the transistor 90 andthe resistor 110 to the circuit ground thereby providing a positive biasvoltage to the base of the motor driver transistor 100. This causes thetransistor to become conductive and thereby supply a voltage from the+24 volt source through the collector load resistor 114 and thecollector-emitter junction of the transistor 100 to the motor relay 118through the semiconductor diode 116. The current flowing through themotor relay 118 energizes it and closes the contact switch 122 tothereby turn the record/playback unit motor on.

The positive voltage supplied to the motor relay 118 at the emitterelectrode of the motor driver transistor 100 is also supplied to theexternal terminal 13e of the dictating station. The resistances 106 and102 in series with the 44 volt source 104 are sufficiently high inmagnitude that the potential developed at the external terminal 132after the motor driver transistor 100 becomes conductive issubstantially +24 volts. The +24 volt potential at terminal 132 flowsthrough the diodes 84 and the in use lights 86 of each of the dictatingstations which are connected in parallel with the dictating station 14.

This positive bias voltage at terminal 13e also prevents any otherdictating stations from seizing control of the system. When the cradlepickup switch 76 in any dictating station is closed subsequent to theseizure of control by another dictating station there is no -44 voltbias voltage to be applied to the emitter electrode of the transistor70. This prevents the transistor pairs 64 and 70 from locking up on eachother and, as will be explained in greater detail, thereby prevents suchother dictating stations from seizing control of the system.

The microphone 26 of the dictating station 14 supplies audio signals tothe transistor pre-amplifier comprised of the transistors 32, 42 and 46which amplify the audio signals and feed the signals through the lead12b to the dictate/playback audio circuit 128 in the record/playbackunit 10 when the dictating switch 56 is closed. Closing of the dictatingswitch 56 connects the audio amplifier circuit of the microphone 26 tothe common return lead 52 which is connected to the circuit groundthrough the emitter-base junction of transistor 64 in parallel with theresistor 68, the collectoremitter junction of transistor 70, the closedcradle pickup switch 76, and the parallel circuit of diode 78 and thecapacitor 80. The closing of the dictate switch 56 also provides aground return path for dictate portion of the control circuit 126through the lead 12b, the collector-emitter junction of the transistor46, the resistor 54, the switch 56, the common return lead 52, theemitter-base junction of the transistor 64 in parallel with the resistor68, the collector-emitter junction of the transistor 70, the switch 76and the diode 78.

The other dictating stations connected in parallel with the dictatingstation 14 are disabled from dictating into the record/playback unit solong as dictating station 14 has seized control of the system because inthe other dictating stations the transistors 70 are in theirnon-conductive state, and as explained above, may not be made conductivebecause of the +24 volt bias voltage applied to the lead 120.

In the playback mode of the system signals are supplied from thedictate/playback audio circuits 128 through the lead 12b, the resistor60, and the collectoremitter junction of the transistor 46 to one leadof the playback loudspeaker 48 in the dictating station 14. The otherlead of the loudspeaker 48 is connected in series through the capacitor50, the common return lead 52, the emitter-base junction of thetransistor 64 in parallel with the resistor 68, the collector-emitterjunction of the transistor 70, the cradle pickup switch 76 and the diode78 to the circuit ground. None of the other dictating stations whichhave not seized control of the system can play back the recorded signalbecause their transistors 70 are not conductive and may not be madeconductive. Thus in such other dictating stations the circuit groundreturn path is electrically open. During dictation, signals recordedthrough the microphone 26 are not simultaneously played back on theloudspeaker 48 because a muting circuit (not shown) contained within thedictating/playback audio circuit mutes any playback signal during thedictating function. A sidetone is, however, produced in the speaker 48due to the voltage appearing across the resistor 54 when the dictate 56is closed. This sidetone notifies the. user that the system isoperating.

In a similar manner all of the playback/rewind controls in each of thedictating stations are connected both to the common return lead 52 andto the conduca tor 12a which is connected to a separate input of thedictate/playback/rewind control circuit 126. The playback/rewindcontrols in all of the dictating stations except the dictating stationwhich has seized control of the dictating system are thus disabledbecause they are blocked from being connected through the common returnlead 52 to the circuit ground by the nonconductive transistor 70.

While transistors of certain conductivity types have been describedabove it should be apparent that in other embodiments transistors ofdifferent conductivity types may be substituted with appropriate changesin the polarities of the biasing circuits. Furthermore in still otherembodiments other types of semiconductor electronic switches, such assilicon controlled rectifi ers, by way of example only, may besubstituted individually or for combinations of the switchingtransistors 64, 70, and 100.

The terms and expressions which have been employed here are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions, of excluding equivalents of thefeatures shown and described, or portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is: V

1. A remote unit for controlling a central mechanism of a system of thetype having a plurality of remote units connected to at least one commonterminal at the central mechanism, the remote unit comprising at leastone control circuit, first electronic switch means connected in seriesbetween the control circuit and the common terminal, means at saidcentral mechanism for supplying a first biasing voltage to said firstelectronic switch means to thereby selectively bias the first electronicswitch means into its conductive state, means responsive to theconductive state of the first electronic switch means for supplying asecond biasing voltage to maintain the first electronic switch means inits conductive state, means at said central mechanism responsive to theconductive state of the first electronic switch to disable the means forsupplying the first biasing voltage when the first electronic switchmeans becomes conductive, and means for selectively disabling the meansfor supplying the second biasing voltage.

2. A remote unit as recited in claim 1 wherein the first electronicswitch means includes a first semiconductor device having a firstelectrode, a second electrode and a first control electrode, the meansfor applying the first biasing voltage includes a first voltage sourceof a predetermined polarity, means for applying thefirst biasing voltageto the first electrode, first resistance means connected between thefirst electrode and the first control electrode and means connecting thefirst control electrode to the common terminal, the means for supplyingthe second bias voltage includes a second semiconductor device having athird electrode, a fourth electrode, and a second control electrode, asecond voltage source having a polarity opposite to that of the firstvoltage source, second resistance means connected between the secondelectrode and the second voltage source, the second control electrodebeing connected to the second electrode, the third electrode beingconnected to the second resistance means and the fourth electrode beingconnected to the first control electrode, and the means for selectivelydisabling the means for supplying the second biasing voltage includes aswitch connected in series between the first electrode and the commonterminal.

3. In a dictating station of a record/playback system of the type havinga plurality of dictating stations, all of the dictating stations beingconnected to at least one common terminal at a record/playback unit andeach dictating station having at least one circuit for selectivelycontrolling the record/playback unit, a privacy and seizure circuitcomprising first electronic switch means connected in series circuitbetween the control circuit and the common terminal, first means at saidrecord/playback unit for selectively applying a first bias voltage of apredetermined polarity to the first electronic switch means to make itbecome substantially conductive, second means at said record/playbackunit for supplying a second bias voltage of the opposite polarity,second electronic switch means responsive to the conductive state of thefirst electronic switch means and supplied with the second bias voltagefor supplying the second bias voltage to the first electronic switchmeans to maintain it in its substantially conductive state when thefirst electronic switch means is made substantially conductive by thefirst bias means, and means at said record/playback unit responsive tothe conductivity of said first electronic switch means for removing saidfirst bias voltage from said first electronic switch means.

4. A privacy and seizure control circuit for a dictating station asrecited in claim 3 further comprising a first diode, a first, a second,and a third resistor and a third switch means having two terminals,wherein the first electronic switch means includes a first transistor ofone conductivity type having collector and emitter electrodes one ofwhich is connected to one terminal of the third switch means, its baseelectrode connected through the first resistor to the common terminal,and the other of its collector and emitter electrodes connected to onelead of the second resistor, the third resistor being connected betweenthe base electrode of the first transistor and the one terminal of thethird switch, wherein the second bias means includes a voltage sourceconnected to the other lead of the second resistor, and wherein thesecond electronic switch means includes a second transistor of aconductivity type opposite to that of the first transistor and havingcollector and emitter electrodes one of which is connected to the baseelectrode of the first transistor and the other of its collector andemitter electrodes con nected to the voltage source of the second biasmeans, and its base electrode connected to the of the collector andemitter electrodes of the first transistor which is connected to thesecond resistor, the first diode being connected between the otherterminal of the third switch means and the common terminal with itspolarity oriented such that current which passes through the firsttransistor and third switch means is passed through the first diode ofthe common terminal, the first bias means including a first bias voltagesource which is connected to the terminal of the third switch meanswhich is connected to the first diode.

5. A privacy and seizure control circuit for a dictating station asrecited in claim 4 further comprising means responsive to the flow ofcurrent from the second bias means through the first transistor fordisabling the first biasing means.

6. In a record/playback system of the type having a plurality of remotedictating stations connected to at least one common terminal at acentral record/- playback unit, each remote dictating station includingat least on separate circuit for controlling the record/- playback unit,a privacyand seizure control circuit the second electronic switch meansa second bias voltage of polarity opposite to that of the first biasvoltage to maintain it in a substantially conductive state, thirdelectronic switch means responsive to the conductive state of the secondelectronic switch means and connected between the second bias means andthe second electronic switch means for supplying the second bias voltageto the second electronic switch means when the second electronic switchmeans is made conductive by being connected through the closing of thefirst switch to the first bias means.

7. A privacy and seizure control circuit for a record/- playback systemas recited in claim 6 and further comprising, in the centralrecord/playback unit, fourth electronic switch means responsive to theflow of current from the second bias means to the second electronicswitch means for activating the central record/- playback unit and fordisabling the first bias means as long as the second electronic switchis in its conductive state.

8. A dictating system having a central record/- playback unit and aplurality of remote dictating stations, each of said dictating stationsbeing connected to said central unit, apparatus for permitting a singledictating station to seize said central unit, comprising first biasingmeans at said central unit for extending a first bias voltagesimultaneously to all of said remote dictating stations; second biasingmeans at said central unit for extending a second bias voltagesimultaneously to all of said remote dictating stations; an electronicswitch at each of said dictating stations and adapted to respond to saidfirst bias voltage to be rendered conductive; selective means at each ofsaid dictating stations, each selective means being responsive to theactuation of its associated dictating station to supply said first biasvoltage to said electronic switch; and means at said central unitresponsive to the conductivity of an electronic switch to simultaneouslyremove the first bias voltage extended to all of said dictating stationsand to extend the second bias voltage to all of said dictating stations,said second bias voltage maintaining said conductive electronic switchin its conductive state and preventing the remaining dictating stationsfrom being rendered conductive.

1. A remote unit for controlling a central mechanism of a system of thetype having a plurality of remote units connected to at least one commonterminal at the central mechanism, the remote unit comprising at leastone control circuit, first electronic switch means connected in seriesbetween the control circuit and the common terminal, means at saidcentral mechanism for supplying a first biasing voltage to said firstelectronic switch means to thereby selectively bias the first electronicswitch means into its conductive state, means responsive to theconductive state of the first electronic switch means for supplying asecond biasing voltage to maintain the first electronic switch means inits conductive state, means at said central mechanism responsive to theconductive state of the first electronic switch to disable the means forsupplying the first biasing voltage when the first electronic switchmeans becomes conductive, and means for selectively disabling the meansfor supplying the second biasing voltage.
 2. A remote unit as recited inclaim 1 wherein the first electronic switch means includes a firstsemiconductor devIce having a first electrode, a second electrode and afirst control electrode, the means for applying the first biasingvoltage includes a first voltage source of a predetermined polarity,means for applying the first biasing voltage to the first electrode,first resistance means connected between the first electrode and thefirst control electrode and means connecting the first control electrodeto the common terminal, the means for supplying the second bias voltageincludes a second semiconductor device having a third electrode, afourth electrode, and a second control electrode, a second voltagesource having a polarity opposite to that of the first voltage source,second resistance means connected between the second electrode and thesecond voltage source, the second control electrode being connected tothe second electrode, the third electrode being connected to the secondresistance means and the fourth electrode being connected to the firstcontrol electrode, and the means for selectively disabling the means forsupplying the second biasing voltage includes a switch connected inseries between the first electrode and the common terminal.
 3. In adictating station of a record/playback system of the type having aplurality of dictating stations, all of the dictating stations beingconnected to at least one common terminal at a record/playback unit andeach dictating station having at least one circuit for selectivelycontrolling the record/playback unit, a privacy and seizure circuitcomprising first electronic switch means connected in series circuitbetween the control circuit and the common terminal, first means at saidrecord/playback unit for selectively applying a first bias voltage of apredetermined polarity to the first electronic switch means to make itbecome substantially conductive, second means at said record/playbackunit for supplying a second bias voltage of the opposite polarity,second electronic switch means responsive to the conductive state of thefirst electronic switch means and supplied with the second bias voltagefor supplying the second bias voltage to the first electronic switchmeans to maintain it in its substantially conductive state when thefirst electronic switch means is made substantially conductive by thefirst bias means, and means at said record/playback unit responsive tothe conductivity of said first electronic switch means for removing saidfirst bias voltage from said first electronic switch means.
 4. A privacyand seizure control circuit for a dictating station as recited in claim3 further comprising a first diode, a first, a second, and a thirdresistor and a third switch means having two terminals, wherein thefirst electronic switch means includes a first transistor of oneconductivity type having collector and emitter electrodes one of whichis connected to one terminal of the third switch means, its baseelectrode connected through the first resistor to the common terminal,and the other of its collector and emitter electrodes connected to onelead of the second resistor, the third resistor being connected betweenthe base electrode of the first transistor and the one terminal of thethird switch, wherein the second bias means includes a voltage sourceconnected to the other lead of the second resistor, and wherein thesecond electronic switch means includes a second transistor of aconductivity type opposite to that of the first transistor and havingcollector and emitter electrodes one of which is connected to the baseelectrode of the first transistor and the other of its collector andemitter electrodes connected to the voltage source of the second biasmeans, and its base electrode connected to the of the collector andemitter electrodes of the first transistor which is connected to thesecond resistor, the first diode being connected between the otherterminal of the third switch means and the common terminal with itspolarity oriented such that current which passes through the firsttransistor and Third switch means is passed through the first diode ofthe common terminal, the first bias means including a first bias voltagesource which is connected to the terminal of the third switch meanswhich is connected to the first diode.
 5. A privacy and seizure controlcircuit for a dictating station as recited in claim 4 further comprisingmeans responsive to the flow of current from the second bias meansthrough the first transistor for disabling the first biasing means. 6.In a record/playback system of the type having a plurality of remotedictating stations connected to at least one common terminal at acentral record/playback unit, each remote dictating station including atleast on separate circuit for controlling the record/playback unit, aprivacy and seizure control circuit comprising in each remote dictatingstation a first switch connected between the control circuit of eachdictating station and the common terminal, second electronic switchmeans connected between the first switch and the control circuit, firstbias means within the central record/playback unit for applying a firstbias voltage of a predetermined polarity through the first switch to thesecond electronic switch means to make it substantially conductuve,second bias means within the central record/playback unit for supplyingto the second electronic switch means a second bias voltage of polarityopposite to that of the first bias voltage to maintain it in asubstantially conductive state, third electronic switch means responsiveto the conductive state of the second electronic switch means andconnected between the second bias means and the second electronic switchmeans for supplying the second bias voltage to the second electronicswitch means when the second electronic switch means is made conductiveby being connected through the closing of the first switch to the firstbias means.
 7. A privacy and seizure control circuit for arecord/playback system as recited in claim 6 and further comprising, inthe central record/playback unit, fourth electronic switch meansresponsive to the flow of current from the second bias means to thesecond electronic switch means for activating the centralrecord/playback unit and for disabling the first bias means as long asthe second electronic switch is in its conductive state.
 8. A dictatingsystem having a central record/playback unit and a plurality of remotedictating stations, each of said dictating stations being connected tosaid central unit, apparatus for permitting a single dictating stationto seize said central unit, comprising first biasing means at saidcentral unit for extending a first bias voltage simultaneously to all ofsaid remote dictating stations; second biasing means at said centralunit for extending a second bias voltage simultaneously to all of saidremote dictating stations; an electronic switch at each of saiddictating stations and adapted to respond to said first bias voltage tobe rendered conductive; selective means at each of said dictatingstations, each selective means being responsive to the actuation of itsassociated dictating station to supply said first bias voltage to saidelectronic switch; and means at said central unit responsive to theconductivity of an electronic switch to simultaneously remove the firstbias voltage extended to all of said dictating stations and to extendthe second bias voltage to all of said dictating stations, said secondbias voltage maintaining said conductive electronic switch in itsconductive state and preventing the remaining dictating stations frombeing rendered conductive.